.\" **************************************************************************
.\" *                                  _   _ ____  _
.\" *  Project                     ___| | | |  _ \| |
.\" *                             / __| | | | |_) | |
.\" *                            | (__| |_| |  _ <| |___
.\" *                             \___|\___/|_| \_\_____|
.\" *
.\" * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
.\" *
.\" * This software is licensed as described in the file COPYING, which
.\" * you should have received as part of this distribution. The terms
.\" * are also available at https://curl.se/docs/copyright.html.
.\" *
.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
.\" * copies of the Software, and permit persons to whom the Software is
.\" * furnished to do so, under the terms of the COPYING file.
.\" *
.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
.\" * KIND, either express or implied.
.\" *
.\" * SPDX-License-Identifier: curl
.\" *
.\" **************************************************************************
.\"
.TH CURLOPT_HTTPHEADER 3 "17 Jun 2014" libcurl libcurl
.SH NAME
CURLOPT_HTTPHEADER \- set of HTTP headers
.SH SYNOPSIS
.nf
#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HTTPHEADER,
                          struct curl_slist *headers);
.fi
.SH DESCRIPTION
Pass a pointer to a linked list of HTTP headers to pass to the server and/or
proxy in your HTTP request. The same list can be used for both host and proxy
requests!

When used within an IMAP or SMTP request to upload a MIME mail, the given
header list establishes the document-level MIME headers to prepend to the
uploaded document described by \fICURLOPT_MIMEPOST(3)\fP. This does not affect
raw mail uploads.

The linked list should be a fully valid list of \fBstruct curl_slist\fP
structs properly filled in. Use \fIcurl_slist_append(3)\fP to create the list
and \fIcurl_slist_free_all(3)\fP to clean up an entire list. If you add a
header that is otherwise generated and used by libcurl internally, your added
one will be used instead. If you add a header with no content as in 'Accept:'
(no data on the right side of the colon), the internally used header will get
disabled. With this option you can add new headers, replace internal headers
and remove internal headers. To add a header with no content (nothing to the
right side of the colon), use the form 'name;' (note the ending semicolon).

The headers included in the linked list \fBmust not\fP be CRLF-terminated,
because libcurl adds CRLF after each header item. Failure to comply with this
will result in strange bugs because the server will most likely ignore part of
the headers you specified.

The first line in an HTTP request (containing the method, usually a GET or
POST) is not a header and cannot be replaced using this option. Only the lines
following the request-line are headers. Adding this method line in this list
of headers will only cause your request to send an invalid header. Use
\fICURLOPT_CUSTOMREQUEST(3)\fP to change the method.

When this option is passed to \fIcurl_easy_setopt(3)\fP, libcurl will not copy
the entire list so you \fBmust\fP keep it around until you no longer use this
\fIhandle\fP for a transfer before you call \fIcurl_slist_free_all(3)\fP on
the list.

Pass a NULL to this option to reset back to no custom headers.

The most commonly replaced HTTP headers have "shortcuts" in the options
\fICURLOPT_COOKIE(3)\fP, \fICURLOPT_USERAGENT(3)\fP and
\fICURLOPT_REFERER(3)\fP. We recommend using those.

There's an alternative option that sets or replaces headers only for requests
that are sent with CONNECT to a proxy: \fICURLOPT_PROXYHEADER(3)\fP. Use
\fICURLOPT_HEADEROPT(3)\fP to control the behavior.
.SH SPECIFIC HTTP HEADERS
Setting some specific headers will cause libcurl to act differently.
.IP "Host:"
The specified host name will be used for cookie matching if the cookie engine
is also enabled for this transfer. If the request is done over HTTP/2 or
HTTP/3, the custom host name will instead be used in the ":authority" header
field and Host: will not be sent at all over the wire.
.IP "Transfer-Encoding: chunked"
Tells libcurl the upload is to be done using this chunked encoding instead of
providing the Content-Length: field in the request.
.SH SPECIFIC MIME HEADERS
When used to build a MIME e-mail for IMAP or SMTP, the following
document-level headers can be set to override libcurl-generated values:
.IP "Mime-Version:"
Tells the parser at the receiving site how to interpret the MIME framing.
It defaults to "1.0" and should normally not be altered.
.IP "Content-Type:"
Indicates the document's global structure type. By default, libcurl sets it
to "multipart/mixed", describing a document made of independent parts. When a
MIME mail is only composed of alternative representations of the same data
(i.e.: HTML and plain text), this header must be set to "multipart/alternative".
In all cases the value must be of the form "multipart/*" to respect the
document structure and may not include the "boundary=" parameter.
.P
Other specific headers that do not have a libcurl default value but are
strongly desired by mail delivery and user agents should also be included.
These are "From:", "To:", "Date:" and "Subject:" among others and their
presence and value is generally checked by anti-spam utilities.
.SH SECURITY CONCERNS
By default, this option makes libcurl send the given headers in all HTTP
requests done by this handle. You should therefore use this option with
caution if you for example connect to the remote site using a proxy and a
CONNECT request, you should to consider if that proxy is supposed to also get
the headers. They may be private or otherwise sensitive to leak.

Use \fICURLOPT_HEADEROPT(3)\fP to make the headers only get sent to where you
intend them to get sent.

Custom headers are sent in all requests done by the easy handles, which
implies that if you tell libcurl to follow redirects
(\fICURLOPT_FOLLOWLOCATION(3)\fP), the same set of custom headers will be sent
in the subsequent request. Redirects can of course go to other hosts and thus
those servers will get all the contents of your custom headers too.

Starting in 7.58.0, libcurl will specifically prevent "Authorization:" headers
from being sent to other hosts than the first used one, unless specifically
permitted with the \fICURLOPT_UNRESTRICTED_AUTH(3)\fP option.

Starting in 7.64.0, libcurl will specifically prevent "Cookie:" headers
from being sent to other hosts than the first used one, unless specifically
permitted with the \fICURLOPT_UNRESTRICTED_AUTH(3)\fP option.
.SH DEFAULT
NULL
.SH PROTOCOLS
HTTP, IMAP and SMTP
.SH EXAMPLE
.nf
CURL *curl = curl_easy_init();

struct curl_slist *list = NULL;

if(curl) {
  curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

  list = curl_slist_append(list, "Shoesize: 10");
  list = curl_slist_append(list, "Accept:");

  curl_easy_setopt(curl, CURLOPT_HTTPHEADER, list);

  curl_easy_perform(curl);

  curl_slist_free_all(list); /* free the list */
}
.fi

.SH AVAILABILITY
As long as HTTP is enabled. Use in MIME mail added in 7.56.0.
.SH RETURN VALUE
Returns CURLE_OK if HTTP is supported, and CURLE_UNKNOWN_OPTION if not.
.SH "SEE ALSO"
.BR CURLOPT_CUSTOMREQUEST "(3), " CURLOPT_HEADEROPT "(3), "
.BR CURLOPT_PROXYHEADER "(3), " CURLOPT_HEADER "(3), "
.BR CURLOPT_MIMEPOST "(3), " curl_mime_init "(3)"
